This disclosure relates to the separation of fiber from cellulose pulp slurry by rotary screening of the pulp stock and, more particularly, to a two stage pressure type screening device. The first stage is a coarse screen with the pulp slurry inflowing a stationary screen and rejecting the coarser particles in the pulp. This first stage is referred to as deknotting in chemical pulping applications, or as simply coarse screening in, for example, old corrugated cardboard (OCC) screening. The second stage is a fine screen to better insure the separation of the rejects from the pulp fibers. Examples of similar devices include U.S. Alajaask Pat. No. 5,575,395 and U.S. Forslund Pat. No. 6,702,120
Two stage screening devices have been known in the past, and three examples of such devices are disclosed in U.S. Hooper Pat. No. 3,898,157, granted Aug. 5, 1975; in U.S. Lamort Pat. No. 3,545,621, granted Dec. 8, 1970; and A. B. Knutsilpalater in Swedish printed Patent application 348,243, filed Feb. 7, 1970. These publications show two screen stages in line on the same vertical axis with the first stage being on top and the screens both being stationary and approximately the same diameter. The pulp slurry inflows through the first stage screen and outflows through the second stage screen. The disclosures show rotating foils within the screens to prevent the perforations or slots from plugging.
U.S. Gero et al. Pat. No. 5,538,632 illustrates a pulp washer, with two concentric, radially spaced apart inside and outside washer screens, with a rotor between the screens, with slurry passing first past the inside screen on the inside surface of the rotor, and then past the outside screen on the outside of the rotor.